Study On The Coupling Effect Of Scavenging Pressure Fluctuation On The Operating Performance Of Free Piston Hybrid System

Under the strategic decision of achieving the dual-carbon goal,the free piston hybrid system(FPHS),as a new type of power plant in recent years,has attracted researchers in the industry due to its advantages of variable compression ratio,strong fuel adaptability,and low energy transfer loss.However,the special operation mode of FPHS will also make the operation performance of the system sensitive to the disturbance of external conditions.The influence of the fluctuation of scavenging pressure on the operation stability and performance of the system is a key problem to be solved urgently.The main reason is that the traditional engine has the guarantee of crank connecting rod mechanism.Even if there is slight disturbance such as scavenging pressure,it can still ensure good ventilation effect and stable operation of the system.However,due to its unique structure,FPHS is more sensitive to the disturbance of external conditions.The fluctuation of scavenging pressure directly affects the trapping ability of FPHS system to fresh charge and the completion degree of cleaning exhaust gas in cylinder,and the effect of ventilation effect on the formation of mixture and combustion performance is obvious.Under such background and research status,this thesis studies the influence of different scavenging pressure characteristics of FPHS on the operation performance of the system.Based on the analysis of the mechanism of mechanical-electric-flow-thermal multi-field coupling,a simulation model considering the interaction between motion,ventilation,spray mixing and combustion was established by using MATLAB/Simulink and AVL-Fire software platforms.The gas exchange,spray mixing and combustion processes of FPHS were simulated,and the gas circulation of the scavenging structure and the accuracy of the spray and combustion models were verified.The influence of scavenging pressure fluctuation on the motion state,ventilation performance,mixture uniformity,combustion performance and emission of FPHS system during the whole operation process was studied and analyzed.The conclusions are as follows:(1)Neither too high nor too low scavenging pressure can provide FPHS with longer piston travel distance and larger compression ratio.Excessive scavenging pressure makes the combustion worse,which is not conducive to the stable and efficient movement of the piston assembly,while the piston movement speed under low scavenging pressure is small.Therefore,the combination of compression ratio and piston motion state to select a moderate scavenging pressure is the key conclusion obtained in this thesis.(2)Moderate scavenging pressure can provide sufficient intake duration.Increasing scavenging pressure will significantly improve the quality of intake flow,but the loss of fresh charge is the largest,which is not conducive to the improvement of trapping efficiency.In addition,the higher scavenging pressure is beneficial to the discharge of exhaust gas in the cylinder and weakens the back-flow of exhaust gas.The improvement of scavenging efficiency is positively correlated with the increase of scavenging pressure,while the trapping efficiency is opposite.(3)Smaller scavenging pressure usually has longer jet penetration,larger SMD,faster fuel evaporation rate,and more fuel evaporation quality.However,the larger scavenging pressure does not support the above results.At low scavenging pressure,the ignition delay time is long,and the fuel atomizing,evaporation and mixing time are long,resulting in more fuel evaporation quality in the ignition delay time and better fuel-air mixing effect.(4)Under low scavenging pressure,the combustion thermal efficiency is high and the combustion is incomplete.Under high scavenging pressure,the after-burning period is short and the reliability is good.At medium scavenging pressure,the combustion cycle time is short and the combustion reaction speed is fast,so the thermal efficiency is high.The mass fraction of NO in the exhaust gas is negatively correlated with the scavenging pressure,but in order to reduce the emission of CO and soot,the scavenging pressure should not be too large.